1. Field of the Invention
This invention relates to surface sealants for welded joints in general, and in particular, relates to polymer fay surface sealants for friction stir welding (FSW) joints used in aircraft construction.
2. Background of the Invention
Welded joints, such as lap joints, are used to join metal parts. Lap joints are used to join overlapping surfaces using one or more welds at the overlap of the materials. The surfaces of each piece of joined metal adjacent the fasteners or welds that are mated by the lap joint, called “fay” surfaces, are often not fully bonded by the weld or other fasteners and are often protected from corrosion by conventional fay surface sealants, such a polysulfide or polythioether, applied to the mating surfaces prior to joining. Such fay surface sealants may also be used to reduce mechanical and fatigue problems resulting from rubbing between the fay surfaces, vibration and the like.
Fay surface sealants are used in welding to prevent or reduce corrosion in large part by reducing moisture which may be trapped between the fay surfaces and/or brought in by capillary action. Conventional fay surface sealants may degrade the properties of the weld and are often degraded by the heat and mechanical activities involved in the welding and working of the materials as well as by vibration during use. Corrosion of fay surfaces within welds is often very difficult to detect by inspection.
Welding techniques typically use elevated temperatures to bond metals and metal alloys. Friction stir welding (FSW) is a welding technique in which the shoulder of a rotating tool is applied to the materials to be joined to heat and soften the materials by friction. The tool includes a rotating pin which penetrates the joint and stirs the materials together. Solid state joints are produced thereby without the addition of filler or the use of shielding gases. Friction stir welding is advantageously used in aircraft construction, for example, to weld a stringer or other support to the surface of the aircraft skin. Conventional FSW welding processes apply a zone of corrosion resistant material to cover the welded joint during and after welding as a fay surface sealant. An example of this technique is disclosed in U.S. Pat. No. 6,045,028, Martin et al. Corrosion of fay surfaces within a FSW weld in an aircraft is a very dangerous condition because of the reduction in strength of the aircraft structure, and it is very difficult to detect by inspection.
What is needed is a new fay surface sealant technique for producing FSW welded components with improved corrosion resistance for use, for example, in the production of airplanes and parts of airplanes.